The order and dynamics of two aromatic polyamides in their lyotropic phases
were investigated with the aid of variable-director nuclear magnetic reson
ance (NMR). In these experiments polymers were dissolved in concentrated su
lfuric acid and allowed to equilibrate inside the main NMR magnetic field B
-0 to yield macroscopically-aligned liquid crystalline solutions. These ord
ered fluids were then rotated away from equilibrium for brief periods of ti
me, and their natural abundance C-13 NMR spectra collected as a function of
different angles between the liquid crystalline director and B-0. The resu
lting spectra showed peaks shifting as well as broadening as a function of
the director's orientation, variations that were also found to be concentra
tion- and temperature-dependent. All such changes could be successfully acc
ounted for on the basis of an exchange model involving molecular reorientat
ions of the polymer chains that are occurring in the intermediate NMR time
scale. Based on this assumption, the experimental line shapes could be used
to extract a detailed description of the macromolecular order and dynamics
in these fluids. The former appeared substantially high, and not very diff
erent from the one characterizing order in commercial extruded aramide fibe
rs. The latter enabled an estimation of the hydrodynamic radii adopted by t
he macromolecules in their mesophases, which ended up in close agreement wi
th dimensions recently reported on the basis of small-angle neutron scatter
ing analyses. (C) 2001 American Institute of Physics.